Yo can start a dummy booking without logging in and begin to type the surname and prename of a passenger. e.g. AB sample

Then the "greyed out" link to the paid seating is coloured and you are able to reserve the seats by row and letter. Non reservable (preferred) seats are greyed out.

It then states: You can book a seat for e.g. 78 Euro.

Just try it and you will see the seat map. Hope this helps.

If not you can go to expedia.co.uk (not expedia.de) and can view the available seats before making the reservation.

Hope this helps. Let us know if this has worked properly.

Otherwise PM the flight numbers and I will look up.

EDIT 14:28 CET Simultaneousely with the great post of shorthauldad, so just keep this advice in mind for the next travels.
Hope you have a safe and convenient journey.